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How to give following BC using GroovyBC

Hi,

I have to give the following boundary condition for pressure
$\frac{dp}{dx} = -\rho\frac{d \phi_g}{dx}$

where

$\phi_g=x$ and it is a variable defined in the code.

I coded it like this

Code:

    side0

    {

    type groovyBC;

    gradientExpression "rho0";

        variables "rho0=-rho*snGrad(phig);";

    value uniform -1.0;

    fractionExpression "0";

    }

But it is giving a wrong value. For

, gradient value at the boundary should be equal to $-\rho$ but it is giving something different.